Olefin polymerization catalyst system is classified into Zeigler-Natta and metallocene catalyst systems, and these high activity catalyst systems have been developed according to each characteristic. Although a Zeigler-Natta catalyst has been widely applied for the existing commercial processes since it was invented in 1950's, since it is a multi site catalyst having a plurality of active sites, the molecular weight distribution of the prepared polymer is wide, and the composition distribution of comonomers is not uniform, and thus, it has a limitation in securing desired properties.
Meanwhile, a metallocene catalyst consists of a main catalyst including a transition metal compound as a main component and a cocatalyst of an organic metal compound including aluminum as a main component, and such a catalyst is a homogeneous complex catalyst and a single site catalyst, and thus, a polymer having narrow molecular weight distribution and uniform composition distribution of comonomers is obtained due to the properties of a single site, and the stereoregularity, copolymerization properties, molecular weight, crystallization degree and the like may be changed by modifying the ligand structure of the catalyst and changing polymerization conditions.
U.S. Pat. No. 5,914,289 describes a method of controlling the molecular weight and the molecular weight distribution of polymer using a metallocene catalyst supported in each carrier, however, a large amount of the catalyst is used for preparing the supported catalyst and the preparation takes long time, and the metallocene catalyst used should be respectively supported in a carrier, which is cumbersome.
Korean Patent Application No. 2003-12308 discloses a method of controlling molecular weight distribution by supporting a dinuclear metallocene catalyst and a mononuclear metallocene catalyst in a carrier together with an activator and polymerizing while changing the combination of the catalysts in a reactor. However, this method has a limit in simultaneous realization of the properties of each catalyst, and has a disadvantage in that metallocene catalyst parts are separated in the carrier component of the finished catalyst to induce fouling of a reactor.
Thus, in order to solve the above disadvantages, there is a continued demand for a method for preparing olefin polymer having desired properties by conveniently preparing a hybrid supported metallocene catalyst having excellent activity.
Meanwhile, linear low density polyethylene is prepared by copolymerizing ethylene and alpha olefin at low pressure using a polymerization catalyst, and has narrow molecular weight distribution and short chain branches of a specific length without long chain branches. A linear low density polyethylene film has high breaking strength and elongation, excellent tear strength, falling weight impact strength and the like, together with general properties of polyethylene, and thus, the use is increasing in a stretch film, an overlap film and the like, to which the existing low density polyethylene or high density polyethylene are difficult to apply.
However, most linear low density polyethylenes using 1-butene or 1-hexene as comonomers are prepared in a single gas phase reactor or a single loop slurry reactor, and although the productivity is higher compared to the process using 1-octene comonomers, the properties of such products are significantly inferior to those using 1-octene comonomer due to the limitations of catalyst technology and process technology, and the molecular weight distribution is narrow and processibility is poor. There have been many efforts to improve these problems.
U.S. Pat. No. 4,935,474 describes a method of preparing polyethylene having wide molecular weight distribution using two or more kinds of metallocene compounds. U.S. Pat. No. 6,828,394 discloses a method of preparing polyethylene having excellent processibility and particularly suitable for a film, using compounds having good comonomer bonding property and compounds lacking in comonomer bonding property in a mixture. Further, U.S. Pat. Nos. 6,841,631, and 6,894,128 state that polyethylene having bimodal or multi-modal molecular weight distribution is prepared with a metallocene catalyst using at least two kinds of metal compounds, and it can be applied for film, blow molding, a pipe and the like. However, although the processibility of these products are improved, distribution state according to molecular weight is not uniform in the unit particle, and the extrusion appearance is rough and the properties are not stable even under relatively good extrusion conditions.
Under these circumstances, there is a continued demand for the preparation of more excellent products balanced between properties and processibility, and particularly, improvement in environmental stress crack resistance is further required.